System for correcting DC characteristics of a magnetic recording system

ABSTRACT

According to an aspect of the present disclosure, a system for correcting for DC characteristics of a magnetic recording system includes: circuitry implementing at least a portion of a write channel of the magnetic recording system; and circuitry configured to process output data of the write channel circuitry in accordance with a read channel of the magnetic recording system and repeatedly trigger re-writing through the write channel circuitry using different ones of a plurality of available data scramblings until a measured baseline wander exceeds a target threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims priority to,U.S. patent application Ser. No. 13/350,684, filed on Jan. 13, 2012,entitled “Method and Apparatus to Limit DC-Level in Coded Data”, nowU.S. Pat. No. 8,358,479, which is a continuation application of, andclaims priority to, U.S. patent application Ser. No. 12/022,131, filedon Jan. 29, 2008, entitled “Method and Apparatus to Limit DC-Level inCoded Data”, now U.S. Pat. No. 8,098,447, which is a divisionalapplication of U.S. patent application Ser. No. 10/752,817, filed onJan. 6, 2004, entitled “Method and Apparatus to Limit DC-Level in CodedData”, now U.S. Pat. No. 7,330,320, which claims priority from U.S.Provisional Patent Application No. 60/478,869, filed on Jun. 16, 2003,entitled “Method and Apparatus to Limit DC-Level in Coded Data”, andU.S. Provisional Application No. 60/485,216, filed on Jul. 7, 2003,entitled “Scrambling to Reduce DC-Content in Encoded Data”. Theapplication herein claims the benefit of priority of all of the abovelisted patent applications and hereby incorporates by reference in theirentirety the said patent applications.

BACKGROUND

Perpendicular magnetic recording (PMR) techniques may enable higherrecording densities on magnetic storage media than conventionallongitudinal magnetic recording techniques. PMR systems include headsthat record bits perpendicular to the plane of the disk. PMR disksinclude a high permeability (“soft”) magnetic underlayer between aperpendicularly magnetized thin film data storage layer and thesubstrate. An image of the magnetic head pole created by the head isproduced in the magnetically soft underlayer. Consequently, the storagelayer is effectively in the gap of the recording head, where themagnetic recording field is larger than the fringing field produced by alongitudinal magnetic recording (LMR) head. The larger recording fieldmakes it possible to record using smaller grain sizes and smaller bitsizes than in LMR systems.

In PMR, the channel response has a DC component. For a channel that isAC-coupled to the preamplifier and read channel, or that contains someother means for high-pass filtering the channel response, there may beDC-distortion. The DC-distortion may manifest itself as a data dependentbaseline wander, which can severely affect the performance of a systemthat equalizes the channel response to a response target that is notDC-free.

SUMMARY

In an embodiment, a perpendicular magnetic recording (PMR) system mayscramble an input data sequence with a first scramble seed and encodethe scrambled data sequence with a modulation encoder (e.g., a runlength limited (RLL) encoder). The system may then determine whether thescrambled and encoded data sequence includes one or more patternsassociated with large baseline wander, using, e.g., the running digitalsum (RDS) over the sequence or a low pass filter as a metric. If such apattern is detected, the system may control the scrambler to re-scramblethe data sequence with another scrambler seed, encode the re-scrambledsequence, and determine whether this scrambled and encoded sequenceincludes on or more patterns associated with large baseline wander. Thisprocess may be repeated until a scrambled and encoded sequence withoutsuch patterns is generated, or until all available scrambler seeds areexhausted, in which case, the scrambler seed with the least amount ofbaseline wander may be used. The best scrambled and decoded sequence maythen be written to the magnetic recording medium.

In an alternative embodiment, a PMR system may scramble an input datasequence with a first scramble seed and encode the scrambled datasequence with a modulation encoder (e.g., a run length limited (RLL)encoder). A write signal may be generated and the encoded data sequencewritten to the magnetic recording medium. The write signal may also befed back to a read channel in the system. The write signal may be passedthrough a filter to mimic the magnetic recording channel. The readchannel may then determine if DC-wander in the write signal is too largeto be accurately decoded. If so, the data sequence may be scrambled withanother scramble seed, encoded, and used to over-write the first encodeddata sequence in the magnetic recording medium. This process may berepeated until an encoded data sequence that can be accurately decodedby the read channel is generated.

For both of the alternative embodiments described above, the datasequence may not be scrambled in the first run through the system.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a read/write head and media in aperpendicular magnetic recording (PMR) system.

FIG. 2 is a block diagram of a write channel and a read channel in thePMR system.

FIG. 3 is a flowchart describing a DC-wander correction techniqueaccording to an embodiment.

FIG. 4 is a flowchart describing a DC-wander correction techniqueaccording to another embodiment.

FIG. 5 is a block diagram of an encoding portion of a write channel in aPMR system.

FIG. 6A is a block diagram of a system modeling DC-offset due to highpass filters.

FIG. 6B is a block diagram of an equivalent system modeling DC-offsetdue to high pass filters using a low pass filter.

FIG. 7 is a block diagram of a discrete time model of a magneticrecording system.

FIG. 8 is a block diagram of a discrete time model of DC-offset in amagnetic recording system.

FIG. 9 is block diagram of a simplified model of DC-offset in a magneticrecording system.

FIG. 10 is block diagram of a simplified model of DC-offset in amagnetic recording system using an adaptive DC-correction circuit.

FIG. 11 is a block diagram of another simplified model of DC-offset in amagnetic recording system using an adaptive DC-correction circuit.

DETAILED DESCRIPTION

FIG. 1 shows a read/write head 102 and magnetic storage disk 104 in aperpendicular magnetic recording (PMR) system. The head records bitsperpendicular to the plane of the disk. PMR disks include a highpermeability (“soft”) magnetic underlayer 106 between a perpendicularlymagnetized thin film data storage layer 108 and the substrate 110. Animage of the magnetic head pole created by the head 102 is produced inthe magnetically soft underlayer 106. Consequently, the storage layer108 is effectively in the gap of the recording head, where the magneticrecording field is larger than the fringing field produced by alongitudinal magnetic recording (LMR) head.

In PMR, the channel response has a DC component. For a channel that isAC-coupled to the preamplifier and read channel, or that contains someother means for high-pass filtering the channel response, there may beDC-distortion. The DC-distortion may manifest itself as a data dependentbaseline wander, which can severely affect the performance of a systemthat equalizes the channel response to a response target that is notDC-free.

FIG. 2 shows a write channel 202 and a read channel 204 for the PMRsystem. In an embodiment, the data that is being written by the writechannel 202 is fed back into the read channel 204. The read channelprocesses the data and decides if the written sequence is likely to havevery poor DC characteristics. If that is the case, the write channelchanges a scrambler seed and rewrites the data using the new scramblerseed.

FIG. 3 is a flowchart describing a DC-wander correction techniqueaccording to an embodiment. A scrambler module 206 may use a scramblerseed 208 to scramble the data 210 input to the write channel 202 (block302). The data may be scrambled before it is encoded by the modulationencoder, e.g., a run length limited (RLL) encoder 212 (block 304).Alternatively, the scrambling may be performed after modulation encodingif the scrambler used does not destroy the constraints imposed by themodulation encoder (for example, if only bits that are left uncoded bythe modulation encoder are scrambled and those bits do not affect howthe encoded bits were encoded).

A number of different scramblers may be used. For example, in anembodiment, a pseudo-noise (PN) sequence generated using amaximum-length shift register may be added modulo-2 to each data bit tobe scrambled. Regardless of the type of scrambler used, the detectormust know if and how the data was scrambled to properly descramble thedata. In an embodiment, the detector may know how the data is scrambled,but may not necessarily know the initial conditions or the scramblerseed that was used to scramble the data. This information (e.g., thescrambler seed or method) may be embedded in the data that is written.Alternatively, the detector may descramble the data by trial and error.For example, the detector may descramble the data following apredetermined list of scramblers/scrambler seeds until the descrambleddata decodes properly by some error-control code (ECC) 213, cyclicredundancy check (CRC) code, or some other check. In an embodiment, thescrambling is done prior to ECC encoding in the write channel, anddescrambling is done after ECC decoding at the detector.

The write signal generated by the write channel is written to the disk(block 305). The write signal is also fed back into the read channel 204(block 306), possibly via a filter 214 to mimic the magnetic channel.The read channel 204 processes the signal (block 308), and a decisionblock 220 determines if the DC-wander is too severe to be handled in theread channel (block 310). If the DC-wander is determined to be withinacceptable limits at block 310, then the next data sequence in the inputdata stream is scrambled (block 312) and encoded. However, if theDC-wander is determined to be too severe, the read channel requests thatthe sector be rewritten with another scrambler seed (block 314). Thenewly scrambled data sequence is then encoded (block 316) and re-writtento the disk (block 318), over-writing the “bad” sequence.

In an embodiment, all of the functions in the read channel 204 thatwould be expected to be active in the actual reading of a waveform fromthe disk are active. An error signal generated internally in the readchannel may be used to monitor how severe the DC-wander is at thedetector input (at block 310). If the DC-wander is determined to be toolarge, a re-write request may be asserted by the read channel. In otherembodiments, various functions of the read channel may disabled. Forexample, in an embodiment, the bit detector may be disabled, since thebits can be obtained directly from the write channel.

Several parameters may be used by the decision block 220 to determinewhether the DC-wander is too large. The following are exemplaryparameters for determining excessive DC-wander:

(a) Simple threshold: the decision block 220 considers the DC-wander tobe too large if the absolute value of the error signal is larger than agiven threshold at any point in the data sequence;

(b) The decision block 220 considers the DC-wander to be too large ifthe absolute value of the error signal is larger than a given thresholdfor a total of at least a given number of clock cycles;

(c) The decision block 220 considers the DC-wander to be too large ifthe absolute value of the error signal exceeds the given threshold forat least a given number of consecutive clock cycles. For example, in anembodiment, the threshold is 3 and the given number of consecutivecycles is three. The error sequence |e|={0, 1, 4, 5, 1, 6, 3, 4, 6, 7,2, 3, 7} has seven numbers greater than 3. However, only the threeconsecutive occurrences of numbers greater than 3 (i.e., thesub-sequence {4, 6, 7}) are counted.

The parameters described above may be used separately or combined. Forexample, the “simple threshold” (a) and “consecutive clock cycle” (c)parameters can be combined. Then the threshold for (a) should be largerthan the threshold for (c).

In an embodiment, an encoded data sequence may be inspected for patternsthat might cause large baseline wander before being written to disk,i.e., in the write channel. The data sequence may be repeatedlyscrambled and encoded until an acceptable level of estimated DC-wanderhas been achieved. The data sequence may then be written to disk.

FIG. 4 is a flowchart describing a DC-wander correction techniqueaccording to an embodiment. As shown in FIG. 5, a scrambler module 502may use a scrambler seed 504 to scramble data sequences in an input datastream 506 (block 402). Information about the scrambler (e.g., thescrambler seed or method) may be embedded in the data so that the datacan be readily decoded by the detector. The scrambled data may then beencoded by an RLL encoder 508 (block 404).

After the RLL encoder, the encoded data may be output to an outputbuffer 510 and a DC-wander estimation module 512 (block 406). TheDC-wander estimation module may screen for patterns that might causelarge baseline wander (i.e., “bad” patterns) (block 408). If no suchpattern is found, the output buffer may output the encoded data to thewrite channel for further processing and writing to the disk (block410). Otherwise, the data is scrambled using another scrambler seed(block 412) and then encoded by the RLL encoder 508 (block 404). Thenewly encoded data sequence is then screened for patterns that may causelarge baseline wander (block 408). This process may continue until theencoded data sequence is determined to contain no bad patterns. Theencoded data is then output to the write channel. In the case that allscrambler seeds are exhausted (block 414), the data may be scrambledusing the scrambler seed that yielded the least baseline wander (block416).

Several metrics may be used by the DC-wander estimation module 512 tomeasure baseline wander. In an embodiment, the maximum absolute value ofthe running digital sum (RDS^(max)) over the entire sequence may beused.

The running digital sum of a binary sequence x={x₀, x₁, . . . , }, wherex_(i)=±1 is defined as

$\begin{matrix}{{{RDS}(n)} = {\sum\limits_{i = 0}^{n}x_{i}}} & (1)\end{matrix}$

The maximum absolute value of RDS over a sequence of length N is

$\begin{matrix}{{RDS}^{\max} = {\max\limits_{0 \leq n \leq {N - 1}}{{{{RDS}(n)}}.}}} & (2)\end{matrix}$

A first method to determine if a sequence has large baseline wander usesequation (2) for the entire sequence. If RDS^(max) is above a certainthreshold, the sequence is considered to have a large baseline wander.In the case that all available sequences have a large baseline wander,the sequence with the smallest RDS^(max) may be selected.

A second method splits the sequence into two or more subsequences, anduses the first method for each sub-sequence. In the case that allavailable sequences have one or more sub-sequences with large baselinewander, the sequence with the smallest number of sub-sequences withlarge baseline wander may be selected. A tie among those can, forexample, be broken by selecting the sequence with the smallestRDS^(max).

For a third method, the number of bit periods for which the absolutevalue of the RDS is greater than a threshold is counted.

$\begin{matrix}{{{RDS}^{count} = {\sum\limits_{i = 0}^{N - 1}I_{i}}},} & (3)\end{matrix}$

where I_(i) is an indicator function

$\begin{matrix}{I_{i} = \left\{ \begin{matrix}{{1\mspace{14mu}{if}\mspace{14mu}{{{RDS}(i)}}} > t_{h}} \\{{0\mspace{14mu}{if}\mspace{14mu}{{{RDS}(i)}}} \leq t_{h}}\end{matrix} \right.} & (4)\end{matrix}$

If RDS^(count) is greater than a certain value, the sequence isdetermined to have a large baseline wander. If all available sequenceshave large baseline wander, then the sequence with the smallestRDS^(count) is selected.

A fourth method is similar to the third method, but only the firstinstance when several consecutive values of the RDS is greater than athreshold is counted.

$\begin{matrix}{{{RDS}^{nbr} = {\sum\limits_{i = 0}^{N - 1}I_{i}^{b}}},{where}} & (5) \\{I_{i}^{b} = \left\{ \begin{matrix}{{1\mspace{14mu}{if}\mspace{14mu}{{{RDS}(i)}}} > {t_{h}\mspace{14mu}{and}\mspace{14mu}{{{RDS}(i)}}} \leq t_{h}} \\{0\mspace{14mu}{otherwise}}\end{matrix} \right.} & (6)\end{matrix}$

For example, if the RDS for a sequence of length 20 is given by {1, 0,−1, −2, −3, −4, −5, −4, −3, −4, −3, −2, −1, 1, 2, 1, 2, 3, 2}, and thethreshold t_(h)=3. Then RDS^(coount)=4 using equation (3), andRDS^(nbr)=2 using equation (5).

In a fifth method, the mean of the absolute value of the RDS is used.

$\begin{matrix}{{RDS}^{mean} = {\frac{1}{N}{\sum\limits_{i = 0}^{N - 1}\;{{{{RDS}(i)}}.}}}} & (7)\end{matrix}$

If RDS^(mean) is greater than a threshold, the baseline wander may beconsidered to be large.

In an alternative embodiment, a low pass filtered version of thesequence is used as a metric rather than the RDS of the sequence. Themain source of baseline wander in many systems is AC coupling or otherhigh pass filtering circuits. The amount of baseline wander caused by acode sequence can be estimated by passing the sequence through a modelof the high pass filter 602 (which mimics AC-coupling), and subtractingthe output of the filter from the input sequence, as shown in FIG. 6A.Equivalently, the amount of baseline wander caused by a code sequencecan be estimated by passing the code sequence through a low pass filter604 with a transfer function that complements the high pass filter, asshown in FIG. 6B. For example, if the high pass filter model has thetransfer function H(z), then the low pass filter should have thetransfer function F(z)=1−H(z).

Other, usually more complex, filters can be designed to mimic not onlythe impact of high pass filters, but also the impact of the write andread process and the signal shaping in the read channel. FIG. 7 shows asimple block diagram of a discrete time model of a magnetic recordingchannel. This model may be used as a basis for the DC-offset model shownin FIG. 8. Since the correct bit decisions are know, they may be usedinstead of a Viterbi detector 702. The DC-offset may be estimated bypassing the encoded sequence through an ideal target response 802 andthen subtracting the finite impulse response (FIR) equalizer output 704from the target response output. This model can be simplified as shownin FIG. 9, where the filter 900 is given byH(D)=t(D)−h(D)g1(D)g2(D)f(D).

Although several examples have been given, any filter that generates anestimate of the DC-offset based on the encoded data sequence as inputcan be used for this purpose.

The following are exemplary parameters that may be used to determine ifa sequence has a large DC-wander in systems using low pass filteredsequences as a metric:

If the largest DC-offset of the sequence is larger than a threshold;

If the largest DC-offsets of the sub-sequences are larger than athreshold;

If the DC-offset is larger than a threshold for more than anotherthreshold number of bit-cycles;

If the DC-offset is larger than a threshold for more than anotherthreshold number of times;

If the mean of the absolute value of the DC-offset is greater than athreshold.

In another embodiment, a low pass filtered sequence with DC correctionis used as a metric. Instead of just estimating the DC-offset based onfiltering the encoded sequence, this method assumes that there is aDC-correction circuit built into the system. Different types ofDC-correction circuits can be used. In FIG. 10, a block diagram of thefilter is shown with an adaptive DC-correction circuit 1000. In mostcases, the DC-correction circuit can be modeled as a low pass filter900. Another block diagram of the DC-offset estimation filter withDC-correction circuit 1100 is shown in FIG. 11. Here, the DC-correctioncircuit uses the DC-corrected channel estimation filter output 1102 asits input.

In an alternative embodiment, several seeds may be used to scramble theencoded sequence in parallel. The scrambled sequences may then beencoded and evaluated in parallel. The seed that provides the bestresponse may then be selected.

In other alternative embodiments, the first trial may not be scrambled.For example, block 302 and 402 in FIGS. 3 and 4, respectively, may beskipped when the sequence is first input to the write channel.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. For example, blocks in theflowcharts may be skipped or performed out of order and still producedesirable results. Accordingly, other embodiments are within the scopeof the following claims.

What is claimed is:
 1. A system for correcting for DC characteristics ofa magnetic recording system comprising: circuitry implementing at leasta portion of a write channel of the magnetic recording system forwriting to the magnetic recording system; and circuitry configured toprocess output data of the write channel circuitry in accordance with aread channel of the magnetic recording system and repeatedly triggerre-writing through the write channel circuitry using different ones of aplurality of available data scramblings until a measured baseline wanderexceeds a target threshold.
 2. The system of claim 1, wherein thecircuitry configured to process the output data comprises a DC-wanderestimation module configured to model the read channel of the magneticrecording system, and the system is configured to use one of theavailable data scramblings that yielded a least baseline wander when allthe available data scramblings are exhausted.
 3. The system of claim 1,wherein the circuitry configured to process the output data comprises aDC-wander estimation module configured to model the read channel of themagnetic recording system, and the DC-wander estimation module isconfigured to use two or more metrics.
 4. The system of claim 1, whereinthe circuitry configured to process the output data comprises aDC-wander estimation module configured to model the read channel of themagnetic recording system, and the DC-wander estimation module isconfigured to use a metric comprising a maximum absolute value of arunning digital sum (RDS) over at least a portion of a sequence for theoutput data.
 5. The system of claim 4, wherein the DC-wander estimationmodule is configured to use the metric comprising the maximum absolutevalue of the RDS over an entirety of the sequence.
 6. The system ofclaim 1, wherein the circuitry configured to process the output datacomprises a DC-wander estimation module configured to model the readchannel of the magnetic recording system, and the DC-wander estimationmodule is configured to use a metric comprising a number of bit periodsan absolute value of a running digital sum (RDS) is greater than athreshold.
 7. The system of claim 1, wherein the circuitry configured toprocess the output data comprises a DC-wander estimation moduleconfigured to model the read channel of the magnetic recording system,and the DC-wander estimation module is configured to use a metriccomprising a mean value of an absolute value of a running digital sum(RDS).
 8. The system of claim 1, wherein the circuitry configured toprocess the output data comprises a DC-wander estimation moduleconfigured to model the read channel of the magnetic recording system,and the DC-wander estimation module is configured to use a metriccomprising an absolute value of a low pass filtered signal generatedfrom the output data.
 9. The system of claim 8, configured to add aDC-correction to the low pass filtered signal.
 10. The system of claim1, wherein the circuitry configured to process the output data comprisesa DC-wander estimation module configured to model one or more high passfiltering circuits of the magnetic recording system.
 11. The system ofclaim 10, wherein the one or more high pass filtering circuits comprisean AC coupling within the magnetic recording system.
 12. The system ofclaim 11, wherein the AC coupling comprises an AC coupling to apreamplifier and the read channel of the magnetic recording system.